1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and a method for controlling threshold values in a nonvolatile semiconductor memory device, and more particularly, to a method for controlling multi-value data stored in memory cells.
2. Related Art
As nonvolatile semiconductor memory devices such as flash memories have larger capacities at lower costs than ever, the data stored in each memory cell is formed with a larger number of bits than ever. If the threshold voltage corresponding to each data state has a wide distribution in a multi-value memory, the space between each two adjacent states becomes narrower, and it becomes difficult to perform data reading with precision. Therefore, a very narrow distribution is required for each threshold value in the memory cells. For example, JP-A 2004-94987 (KOKAI) discloses a write method for obtaining narrow threshold voltage distributions.
As the memory cells in such a multi-value memory are becoming smaller and smaller, a variation of the threshold value of a memory cell adjacent to a subject memory cell causes the threshold value of the subject memory cell to vary. This is the problem called the adjacency effect.
Also, the adjustment time (the write time) is expected to become shorter, even though very narrow threshold value distributions are required. For example, JP-A 2007-141447 (KOKAI) discloses a write method for increasing the write speed.